The stratum corneum that is the outermost layer of the human skin functions to protect the skin from external harmful environment and harmful materials and interrupts evaporation of the moisture in the human body, thereby functioning as a skin barrier preventing skin dryness. Lipid ingredients play a particularly important role in the stratum corneum. Elias and coworkers have reported that the lamellar structure formed of the lipid ingredients in the stratum corneum is the origin of such a skin barrier function (J. Invest. Dermatol. 69: 535-546, 1977). In addition, although there is a difference depending on the experimental method and the position of a skin tissue, Wertz and coworkers have reported that intercellular lipids substantially include about 50% of ceramides, 20-25% of cholesterol, 20-25% of free fatty acids, about 10% of cholesterol ester, 1-2% of cholesterol sulfate, a small amount of phospholipid, a small amount of phytosphingosine and a small amount of triglycerides (Adv. Drug. Deliv. Rev. 18: 283-294, 1996).
Intercellular lipid ingredients, particularly ceramides and cholesterol, are mostly hydrophobic, highly crystallizable and sparingly soluble materials. They are not dissolved completely into an oil phase that is a dispersion phase in a general oil-in-water emulsion formulation but are partially present in an aqueous phase that is a continuous phase. For this reason, problems related with formulation stability, such as gelation, may occur.
To solve the above-mentioned problem, there has been suggested a stabilization method including forming granules (particles) including intercellular lipids and other ingredients and adding the granules in the last step during the preparation of an emulsion (Korean Patent Application No. 2008-0072335). However, the method is problematic in that the amount of intercellular lipids that may be used actually in a skin application agent is decreased due to an excessive amount of encapsulating base used for an encapsulation step, and an encapsulating agent based on saccharides or polymers causes degradation of the feeling of use of a skin application agent.
As a part of attempts to stabilize intercellular lipids in emulsion, there has been suggested a method including preliminarily forming a liquid crystal base and adding it in the last step during the preparation of a general oil-in-water emulsion (Korean Patent Publication Nos. 10-0452757, 10-0525842, 10-0830153, 10-0867679). However, such a liquid crystal base is present mostly in a wax or paste phase having high hardness at room temperature, and thus shows low compatibility during processing. In addition, the preparation of such a liquid crystal base requires use of an excessive amount of lecithin and higher fatty alcohol. However, such materials undesirably have a stiff feeling of use. Moreover, when using a liquid crystal base, emulsion undergoes an increase in viscosity or hardness with time. It is thought that this is because the surfactant micelles present in the emulsion attack the oil particles of the liquid crystal emulsion obtained from the liquid crystal base, resulting in degradation of the stability of liquid crystal emulsion. Similarly, there has been reported that surfactant particles present in the form of micelles cause mass transfer among the oil particles of oil-in-water emulsion (Food Hydrocolloids, 6(5): 415-422, 1992).
Finally, there has been an attempt to form a liquid crystal emulsion through a direct emulsification process in order to stabilize intercellular lipids in emulsion. Korean Patent Publication No. 10-0588743 discloses a method for preparing a liquid crystal emulsion having a multilayer lamellar structure including a POE-based surfactant, intercellular lipids, higher fatty alcohol, or the like. However, such a liquid crystal emulsion including a POE-based surfactant causes swelling in an emulsion system, thereby affecting the overall stability of an emulsion system (G. M. Eccleston, Miltiple-phase oil-in-water emulsions, J. Soc. Cosmet. Chem., (41) 1-22, 1990). In addition, according to the above document, a phase transition emulsification process using temperature control is applied. In general, a phase transition emulsification process is advantageous in that it produces particles having a uniform size. However, such a process is complicated and requires use of a slow cooling step in a natural cooling mode, and thus has low time efficiency. According to Korean Patent Application No. 2011-0093634, a liquid crystal emulsion is prepared by using a glucoside surfactant, ceramide and oil. However, in this case, a slow cooling mode is applied. According to the above document, it is said that slow cooling provides liquid crystal particles having an adequate size, as compared to rapid cooling that produces very small particles having a low product value. However, according to the study of the inventors of the present disclosure, controlling a particle size depends on a compositional ratio of contents rather than a cooling mode. In addition, another patent document (Korean Patent Publication No. 10-0830153) that belongs to the same category discloses that cetostearyl alcohol, a higher fatty alcohol, present in an oil phase moves toward the interface of particles, while it precipitates in the form of crystals through a cooling step, in a process of forming liquid crystals, and application of rapid cooling at that time prevents cetostearyl alcohol from moving toward the interface and causes it to be present in oil in an oversaturated sate, resulting in a decrease in liquid crystal formation and a drop in viscosity. However, a process of forming liquid crystals does not include precipitation of a higher fatty alcohol that has been dissolved in an oil phase in the form of crystals at high temperature through a cooling step, but includes phase transition of a higher fatty alcohol into a liquid crystal phase having both of solid properties and liquid properties and orientation thereof at the particle interface in a certain direction together with the surfactant. In addition, although the above document discloses that rapid cooling prevents cetostearyl alcohol from moving toward the interface, cetostearyl alcohol itself is an amphiphilic material, and thus is not present in an oil phase but tends to move toward the interface. Therefore, the moving rate of cetostearyl alcohol to the interface may be controlled by the type of oil used in forming liquid crystals and cooling rate. Further, the above document discloses that cetostearyl alcohol is present in an amount equal to or higher than its solubility in oil. However, incorporation of an excessive amount of cetostearyl alcohol may cause precipitation of a higher fatty alcohol and degradation of a feeling of use.
Under these circumstances, the present inventors have conducted active studies to overcome the above-mentioned problems. After the studies, we have found that it is possible to prepare a stable liquid crystal emulsion having a multilayer lamellar structure including a large amount of intercellular ingredients through a rapid cooling process, wherein the liquid crystal emulsion has an oil phase part that contains intercellular lipid ingredients, non-POE based non-ionic surfactant, higher fatty alcohol, oil and wax, and an aqueous phase part that contains water, polyol and the other aqueous phase ingredients. The present disclosure is based on this finding.